Organic Light Emitting Diode (OLED) is considered as next generation of perfect display device due to its advantages of high color saturation, wide visual angle, low energy consumption, high luminance, and simple device structure and so on.
At present, large size of OLED display device is realized, but two problems of non-uniformity and image-retention of display still exist in terms of luminance. The display non-uniformity of luminance mainly results from a non-uniformity of electrics performance of Thin Film Transistor (TFT) in back plane and internal resistance (IR) drop produced from implementation of image drive. The image-retention is a kind of image-retention due to aging OLED and TFT device, which basically results from an attenuation degree of TFT and OLED of a display area due to long time illumination of the display area is greater than the attenuation degree of TFT of another display area. With respect to these two problems, improvement is usually done by adopting a control technology of image display during making image drive.
The method of controlling image display comprises internal compensation and external compensation. The internal compensation means a method of making compensation by a sub circuit constructed with TFT inside a sub-pixel. The external compensation means a method of extracting a TFT and OLED signal out of the back plane and then controlling image display by means of external application specific integrated circuit (ASIC). Sub-pixel structure and driving manner of the internal compensation is generally complicated, the compensation effect is merely limited to TFT threshold voltage and IR Drop, and the problem of image-retention can not be removed. Meanwhile, in large size of display device with high resolution and high refresh frequency, the method of internal compensation results in problems of decreased aperture ratio and slow driving speed. The manner of external compensation has advantages of simple sub-pixel structure, fast driving speed and good compensation effect, and thus external compensation is preferable compensation scheme in large size OLED display.
The external compensation is classified into optical extractable compensation and electrical extractable compensation. The optical extractable compensation refers to extracting luminance signal by means of optical Charge-coupled Device (CCD) photographing after the back plane is illuminated, and the electrical extractable compensation refers to extracting electrical signal of TFT and OLED by means of a induction circuit of a driving chip. The kinds of signal extracted by the two compensation methods are different, and thus data is processed in difference manner. The optical extractable compensation has advantages of simple structure and flexibility, and thus is widely used currently.
The existing method of optically compensating image luminance is performed as follows: comparing luminance signal value obtained by CCD photographing with an ideal luminance signal value, then selecting proper gray scale offset amount ΔG, and making compensation in a manner of progressive approximation. Compensation precision of this method depends on the value of ΔG, and compensation range is (2n−1)ΔG, wherein n is number of measurement. Therefore, ΔG needs to be decreased in order to increase compensation precision, and only the number of measurement can be increased in order to widen the compensation range. What's more, as for every gray scale, many times of measurement and comparison are necessary, and thus it is inefficient. As shown in FIG. 1, FIG. 1 is a relationship graph of number of measurement versus luminance when image display is controlled by existing manner of progressive approximation. Since time taken by measurement is long and the amount of data processing increases, if the number of measurement is reduced, the amount of data processing will be decreased, but a requirement for compensation precision can not be met.